Photonic integrated circuits are applied in many application fields such as in the area of fiber-optic communication, biomedical applications, photonic computing, signal processing, etc. Such photonic integrated circuits typically may comprise a number of optical elements such as a detector, a source, a resonator or another radiation processing element, interconnected through waveguides. For radiation coupling to a photonics integrated circuit, grating coupling still by far is the most commonly used solution.
Proper alignment between the incident radiation beam, e.g. stemming from an optical fiber or another radiation source, and the grating is often crucial for having a good coupling of the radiation into the waveguide. Nevertheless, significant miniaturization of the components of the photonic integrated circuit has occurred during the past years, rendering alignment of the incoming radiation and the device more and more difficult. One example of photonic integrated devices where miniaturization has strongly occurred are photonic integrated devices based on silicon-on-insulator. The high index contrast of SOI allows for example for fabrication of micrometer-size ring resonators which can be used for a variety of photonic applications. Coupling of radiation to the small resonators is performed using small gratings. Using such a system multiplexing can be performed, such as for example multiplexed assay systems for biosensing applications. In order to relax the alignment conditions between the irradiation source and the grating coupler for coupling to the device, flood illumination is sometimes used. When using flood illumination, a large area of the photonic integrated device is irradiated. This technique provides a very high alignment tolerance. At the same time, as useful in some applications, it allows coupling to a plurality of coupling gratings simultaneously for rapid multiplexing. Using such a solution thus allows relaxing the alignment requirements between the incoming radiation and the coupling grating.
However, flood illumination has the disadvantage that radiation that is not incident on the coupling grating but is incident in the area around the coupling grating causes parasitic radiation. Parasitic reflections on the device surface can deteriorate the operation and quality of the photonic integrated circuit and thus limit the performance.
One solution that was offered in the past is the use of a reflective gold film, in the area of the photonics device outside the coupling grating, for reflecting the radiation back into free space. A gold film then is applied as additional layer on top of the photonics device. This however is a costly and additional processing step. There is still room for improvement in systems and methods for coupling radiation into a waveguide of a photonic integrated circuit.